FIG. 1 illustrates a basic structure of the electrostatic capacitive vibration sensor. In a vibration sensor 11, a vibrating electrode plate 13 is disposed in an upper surface of a substrate 12 whose central portion is opened, an upper portion of the vibrating electrode plate 13 is covered with a fixed electrode plate 14, and plural acoustic holes 15 are made in the fixed electrode plate 14. When an acoustic vibration 16 propagates toward the vibration sensor 11, the acoustic vibration 16 vibrates the vibrating electrode plate 13 through the acoustic holes 15. Because a distance between the vibrating electrode plate 13 and the fixed electrode plate 14 changes when the vibrating electrode plate 13 is vibrated, the acoustic vibration 16 (air vibration) can be converted into an electric signal and be output by detecting a change in electrostatic capacitance between the vibrating electrode plate 13 and the fixed electrode plate 14.
In the vibration sensor 11, the acoustic holes 15 perform the following functions:
(1) a function of not applying a sound pressure to a fixed film,
(2) a function of reducing damping of the vibrating electrode plate to improve a high-frequency characteristic, and
(3) a function as an etching hole in preparing an air gap.
The acoustic hole 15 also has a large influence on a function of a vent hole. The functions of the acoustic hole and vent hole will be described below.
(Function of Not Applying Sound Pressure to Fixed Film)
In the vibration sensor 11, the vibrating electrode plate 13 is forcedly vibrated by the acoustic vibration 16 to detect the acoustic vibration 16. When the fixed electrode plate 14 is simultaneously vibrated along with the vibrating electrode plate 13, detection accuracy of the acoustic vibration is degraded. Therefore, in the vibration sensor 11, rigidity of the fixed electrode plate 14 is set higher than that of the vibrating electrode plate 13, and the acoustic holes 15 are made in the fixed electrode plate 14 to cause the sound pressure to escape from the acoustic holes 15, whereby the fixed electrode plate 14 is hardly vibrated by the sound pressure.
(Function of Reducing Damping of Vibrating Electrode Plate to Improve High-Frequency Characteristic)
When the acoustic holes 15 are not made, air is trapped in the air gap 17 (void) between the vibrating electrode plate 13 and the fixed electrode plate 14. Because the trapped air is compressed or expanded according to the vibration of the vibrating electrode plate 13, the vibration of the vibrating electrode plate 13 is damped by the air. On the other hand, when the acoustic holes 15 are made in the fixed electrode plate 14, because the air enters and exits the air gap 17 through the acoustic holes 15, the vibration of the vibrating electrode plate 13 is hardly damped, thereby improving the high-frequency characteristic of the vibration sensor 11.
(Function as Etching Hole in Preparing Air Gap)
In a method for forming an air gap 17 between the fixed electrode plate 14 and the vibrating electrode plate 13 by a surface micromachining technology, a sacrifice layer is formed between the substrate 12 and the vibrating electrode plate 13 or between the vibrating electrode plate 13 and the fixed electrode plate 14. An etching solution is introduced to the inside from the acoustic holes 15 made in the fixed electrode plate 14, and the sacrifice layer is removed by etching to form the air gap 17 between the vibrating electrode plate 13 and the fixed electrode plate 14.
(Relationship Between Vent Hole and Acoustic Hole)
A through-hole or a recess is provided in the substrate 12 so as not to interfere with the vibration of the vibrating electrode plate 13. When the recess (back chamber 18) is provided in an upper surface of the substrate 12, the back chamber 18 is closed on the lower surface side of the substrate. For the through-hole, although the through-hole pierces from the upper surface of the substrate to the lower surface, frequently the lower surface of the through-hole is closed by a wiring substrate by mounting the vibration sensor on the wiring substrate (accordingly, hereinafter the case of the through-hole is also referred to as back chamber 18). Therefore, occasionally a pressure in the back chamber 18 differs from an atmospheric pressure. Occasionally a pressure in the air gap 17 also differs from the atmospheric pressure due to a ventilation resistance.
As a result, a pressure difference is generated between the upper surface side (air gap 17) and the lower surface side (back chamber 18) of the vibrating electrode plate 13 according to a fluctuation in ambient pressure or a change in ambient temperature, and the vibrating electrode plate 13 is bent to possibly become a measurement error of the vibration sensor 11. In the general vibration sensor 11, as illustrated in FIG. 1, a vent hole 19 is made in the vibrating electrode plate 13 or between the vibrating electrode plate 13 and the substrate 12 to communicate the upper surface side and lower surface side of the vibrating electrode plate 13 to each other, thereby eliminating the pressure difference between the upper surface side and the lower surface side.
However, for the large acoustic hole 15 located near the vent hole 19, an acoustic resistance is decreased in a ventilation pathway 20 (indicated by an arrow of FIG. 1) from the acoustic hole 15 to the back chamber 18 through the vent hole 19. Therefore, the low-frequency acoustic vibration entering the vibration sensor 11 through the acoustic hole 15 near the vent hole 19 passes easily through the vent hole 19 to the back chamber 18. As a result, the low-frequency acoustic vibration passing through the acoustic hole 15 near the vent hole 19 leaks onto the side of the back chamber 18 without vibrating the vibrating electrode plate 13, and thereby degrading the low-frequency characteristic of the vibration sensor 11.
As illustrated in FIG. 2, when dust 23 such as dirt and micro particles invades from the acoustic hole 15, the dust 23 is deposited on the air gap or the vent hole. Because generally the vent hole 19 is narrower than the air gap, the vent hole 19 clogs when the dust 23 enters the vent hole 19, which results in interference of the vibration of the vibrating electrode plate 13 or a change in the number of vibrations. Therefore, sensitivity of the vibration sensor or frequency characteristic is possibly degraded.
(Sticking of Electrode Plates)
In the vibration sensor 11 of FIG. 1, occasionally sticking of the electrode plates is generated during use or a production process. The sticking, as illustrated in FIG. 3(b), means a state in which part or substantial whole of the vibrating electrode plate 13 is fixed to the fixed electrode plate 14 and hardly separated from the fixed electrode plate 14. When the vibrating electrode plate 13 sticks to the fixed electrode plate 14, because vibration of the vibrating electrode plate 13 is prevented, the vibration sensor 11 cannot detect the acoustic vibration.
FIGS. 3(a) and 3(b) are schematic diagrams explaining a cause of generation of the sticking in the vibration sensor 11. Because the vibration sensor 11 is produced by utilizing the micromachining technology, for example, moisture w invades between the vibrating electrode plate 13 and the fixed electrode plate 14 in a cleaning process after etching. Even in use of the vibration sensor 11, occasionally the moisture remains between the vibrating electrode plate 13 and the fixed electrode plate 14 or the vibration sensor 11 is wetted.
On the other hand, because the vibration sensor 11 has micro dimensions, there is only a gap of several micrometers between the vibrating electrode plate 13 and the fixed electrode plate 14. Additionally, because the vibrating electrode plate 13 has a thickness of about 1 micrometer in order to enhance the sensitivity of the vibration sensor 11, the vibrating electrode plate 13 has a weak spring property.
Therefore, in the vibration sensor 11, occasionally the sticking is generated through the following two-stage process. In a first stage, as illustrated in FIG. 3(a), when the moisture w invades between the vibrating electrode plate 13 and the fixed electrode plate 14, the vibrating electrode plate 13 is attracted to the fixed electrode plate 14 by a capillary force P1 or a surface tension of the moisture w.
In a second stage, as illustrated in FIG. 3(b), after the moisture w between the vibrating electrode plate 13 and the fixed electrode plate 14 is evaporated, the vibrating electrode plate 13 sticks to the fixed electrode plate 14, and the sticking state is maintained. An intermolecular force, an interfacial force, and an electrostatic force, which act between the surface of the vibrating electrode plate 13 and the surface of the fixed electrode plate 14, can be cited as an example of a force P2 that fixes and maintains the vibrating electrode plate 13 to and in the fixed electrode plate 14 after the moisture w is evaporated. As a result, the vibrating electrode plate 13 is retained while sticking to the fixed electrode plate 14, and the vibration sensor 11 malfunctions.
In the first stage, the vibrating electrode plate 13 sticks to the fixed electrode plate 14 by the capillary force of the invading moisture. However, in some cases, the vibrating electrode plate sticks to the fixed electrode plate by a liquid except the moisture, and the vibrating electrode plate sticks to the fixed electrode plate by applying the large sound pressure to the vibrating electrode plate. Occasionally, the vibrating electrode plate takes on static electricity to stick to the fixed electrode plate, thereby generating the process in the first stage.
(Thermal Noise)
The inventors found that a noise generated in the vibration sensor is caused by a thermal noise (fluctuation of air molecule) in the air gap 17 between the vibrating electrode plate 13 and the fixed electrode plate 14. As illustrated in FIG. 4(a), air molecules α existing in the air gap 17 between the vibrating electrode plate 13 and the fixed electrode plate 14, that is, a quasi-closed space collide with the vibrating electrode plate 13 by the fluctuation, a micro force generated by the collision with the air molecules α acts on the vibrating electrode plate 13, and the micro force acting on the vibrating electrode plate 13 varies randomly. Therefore, the vibrating electrode plate 13 is vibrated by the thermal noise, and an electric noise is generated in the vibration sensor. Particularly, in the high-sensitivity vibration sensor (microphone), the noise caused by the thermal noise is increased to degrade an S/N ratio.
According to knowledge obtained by the inventors, it is found that the noise caused by the thermal noise is reduced by making the acoustic holes 15 in the fixed electrode plate 14 as illustrated in FIG. 4(b). The inventors also obtained the knowledge that the noise is decreased, as an opening area of the acoustic hole 15 is enlarged, and as an interval at which the acoustic holes 15 are disposed is narrowed. This is attributed to the fact that, when the acoustic holes 15 are made in the fixed electrode plate 14, the air in the air gap 17 escapes easily from the acoustic hole 15, and the number of air molecules α colliding with the vibrating electrode plate 13 is decreased to reduce the noise.
(Well-Known Vibration Sensor)
For example, Patent Document 1 (Japanese Unexamined Patent Publication No. 2007-274293) discloses a capacitor microphone that is of the electrostatic capacitive vibration sensor. In the vibration sensor disclosed in Patent Document 1, as illustrated in FIGS. 1 and 2 of Patent Document 1, a vibrating electrode plate (12) (the numeral in parenthesis indicated about the vibration sensor of Patent Document 1 is used as well as in Patent Document 1) is opposite to a fixed electrode plate (3), a vent hole (15) is made in an end portion of the vibrating electrode plate, and acoustic holes (5) having an even size are evenly arrayed in the fixed electrode plate.
However, in the vibration sensor of Patent Document 1, because of the even size of the acoustic hole, when the opening area of the acoustic hole is enlarged, the acoustic hole near the vent hole is enlarged to decrease the acoustic resistance of the ventilation pathway including the vent hole. As a result, unfortunately the low-frequency characteristic of the vibration sensor is degraded.
Additionally, when the opening area of the acoustic hole is enlarged, the dust invades easily from the acoustic holes near the vent hole, and the vent hole clogs easily by the invading dust (see FIG. 2). Therefore, the vibration characteristic of the vibration electrode film varies to easily change the sensitivity or frequency characteristic of the vibration sensor.
On the other hand, in the vibration sensor of Patent Document 1, because the damping suppression effect of the vibrating electrode plate is lowered when the opening area of the acoustic hole is reduced, the high-frequency characteristic of the vibration sensor is lowered. Additionally, when the opening area of the acoustic hole is reduced, because the fixed electrode plate is easily subjected to the sound pressure, accuracy of the vibration sensor is also easy to be lower.
Accordingly, there is a contradictory problem in the vibration sensor of Patent Document 1. That is, when the opening area of the acoustic hole is enlarged, the low-frequency characteristic of the vibration sensor is lowered, or the change of the sensor characteristic is easily increased by the dust. On the other hand, when the opening area of the acoustic hole is reduced, the high-frequency characteristic is lowered, or the sensor accuracy is largely degraded by the fixed electrode plate subjected to the sound pressure.
Further, the sticking problem exists in the vibration sensor that is prepared by utilizing the micromachining technology, and the sticking is correlated with a contact area of the vibrating electrode plate and the fixed electrode plate. Therefore, when the opening area of the acoustic hole is reduced in the vibration sensor of Patent Document 1, unfortunately the sticking of the electrode plates is easy to generate.
According to knowledge obtained by the inventors, when the opening area of the acoustic hole is reduced in the vibration sensor of Patent Document 1, unfortunately the noise caused by the thermal noise of the vibration sensor is increased.
(Another Well-Known Vibration Sensor)
For example, Patent Document 2 (U.S. Pat. No. 6,535,460) discloses another vibration sensor. In the vibration sensor of Patent Document 2, as illustrated in FIGS. 2 and 3 of Patent Document 2, a vibrating electrode plate (12) (the numeral in parenthesis indicated about the vibration sensor of Patent Document 2 is used as well as in Patent Document 2) is opposite to a fixed electrode plate (40), and a void is formed between the vibrating electrode plate and a substrate (30). A circular-ring-shape projected strip (41) is formed in a lower surface of the fixed electrode plate, ventilation holes (21) are made in a circular region located inside the projected strip of the fixed electrode plate, and ventilation holes (14) are made in a circular-ring-shape region located outside the projected strip of the fixed electrode plate. Each opening area in the ventilation hole (21) located inside the projected strip is larger than that of the outside ventilation hole, and the ventilation holes (21) are regularly arrayed at intervals smaller than those of the outside ventilation holes. Each opening area in the ventilation hole (14) located outside the projected strip is smaller than that of the inside ventilation hole, and the ventilation holes (14) are unevenly formed at intervals larger than those of the inside ventilation holes.
However, in the vibration sensor of Patent Document 2, the inner-peripheral-portion ventilation hole (21) provided in the fixed electrode plate differs significantly from the outer-peripheral-portion ventilation hole (14) in the array interval, and the outer-peripheral-portion ventilation holes are unevenly arrayed. Therefore, during producing the vibration sensor, unfortunately an etching required time is unnecessarily lengthened while the etching becomes uneven in a process for etching the sacrifice layer formed between the vibrating electrode plate and the fixed electrode plate.
FIG. 5 illustrates the case in which the acoustic holes 15 (ventilation holes) are unevenly disposed in the vibration sensor 11 of FIG. 1. FIG. 5(a) is a schematic plan view illustrating a state in which a sacrifice layer 22 is being removed by the etching through the unevenly disposed acoustic holes 15, FIG. 5(b) is a sectional view taken on a line X-X of FIG. 5(a), and FIG. 5(c) is a schematic plan view illustrating a state in which the removal of the sacrifice layer 22 is completed by the etching through the unevenly disposed acoustic holes 15.
When the acoustic holes 15 are unevenly disposed as illustrated in FIG. 5(a), because etching solutions invading from the acoustic holes 15 have the same etching rate, the sacrifice layer 22 is unevenly etched, as illustrated in FIG. 5(b), the sacrifice layer 22 is rapidly etched in a region where the interval between the acoustic holes 15 is narrowed, and the sacrifice layer 22 is slowly etched in a region where the interval between the acoustic holes 15 is widened. Therefore, in the region where the interval between the acoustic holes 15 is widened, a time necessary for etching the sacrifice layer 22 is lengthened, and eventually the etching required time is unnecessarily lengthened. In the region where the interval between the acoustic holes 15 is shortened, because the etching is continued even after the sacrifice layer 22 is etched to expose the fixed electrode plate 14 and the vibrating electrode plate 13, an etching degree of the fixed electrode plate 14 becomes large as illustrated in FIG. 5(c). As a result, an uneven stress is applied to the fixed electrode plate 14 even in the middle of the etching process, and possibly the fixed electrode plate 14 breaks. Even if the fixed electrode plate 14 does not lead to the breakage, because of the uneven disposition of the acoustic holes 15, a bias is generated in the etching degree of the fixed electrode plate 14, that is, a partial thickness of the fixed electrode plate 14, which possibly causes a characteristic defect of the vibration sensor.
Accordingly, even in the vibration sensor of Patent Document 2, the bias is generated in the etching degree because of the uneven disposition of the ventilation holes (21 and 14), unfortunately a defect occurrence rate of the vibration sensor is increased or the etching required time is unnecessarily lengthened.
In the vibration sensor of Patent Document 2, the vibrating electrode plate except a wiring lead portion is separated from the substrate, the vibrating electrode plate is sucked onto the fixed electrode plate side by an electrostatic attractive force acting between the vibrating electrode plate and the fixed electrode plate in used of the vibration sensor, and the vibrating electrode plate abuts on the lower surface of the projected strip. Therefore, because the air gap between the vibrating electrode plate and the fixed electrode plate becomes a substantially closed space surrounded by the projected strip, the lower-surface-side space (back chamber) and upper-surface-side space (air gap) of the vibrating electrode plate are partitioned by the projected strip and not communicated with each other although the void is formed between the vibrating electrode plate and the substrate. That is, in the vibration sensor of Patent Document 2, the void between the vibrating electrode plate and the substrate neither functions as the vent hole nor is the vent hole.
Similarly, although the ventilation hole (21) on the inner peripheral side is communicated with the air gap to function as the acoustic hole, the ventilation hole (14) on the outer peripheral side does not function as the acoustic hole because the ventilation hole (14) is not communicated with the air gap. Therefore, only the ventilation hole (21) on the inner peripheral side becomes the acoustic hole in the vibration sensor of Patent Document 2, and the acoustic holes having the even opening area are regularly arrayed in the vibration sensor of Patent Document 2 like the vibration sensor of Patent Document 1.
Further, in the vibration sensor of Patent Document 2, because the vibrating electrode plate is sucked onto the fixed electrode plate side to abut on the lower surface of the projected strip by the electrostatic attractive force, the upper surface of the vibrating electrode plate is retained in or substantially fixed to the lower surface of the projected strip over the whole circumference, and unfortunately the vibration of the vibrating electrode plate is suppressed by the contact with the projected strip to easily lower the sensitivity of the vibration sensor.
Patent Document 1: Japanese Unexamined Patent Publication No. 2007-274293
Patent Document 2: U.S. Pat. No. 6,535,460